It is often necessary or desirable to measure the value of various compositional parameters of blood, i.e., the presence and/or concentration of blood constituents, such as blood gases, hydrogen ions (pH), other electrolytes, glucose, red blood cells and the like. Measurements of the values of compositional parameters of blood can be accomplished electrochemically or optically using fluorescence or absorbance techniques.
Compositional parameters of blood can be monitored in real time using, for example, fluorescent sensors. The sensors occupy positions in a sensor housing or cassette adjacent a blood passageway. The sensors are illuminated with exciting light of one wavelength, and each of them emits a fluorescent emitted signal having an intensity which varies according to the value of the compositional parameter of blood to which the sensor responds. These signals are then processed to provide quantitative information concerning, or values of, the relevant compositional parameters.
After the sensors are exposed to the compositional parameters of blood, the signals emitted by the sensors change from some initial value to a final or equilibrated value over a length of time which may be minutes in duration. The resulting time delay can cause certain problems because the time passing before equilibration occurs may be significant in terms of blood coagulation time. Thus, there is a risk of coagulation in the cassette while waiting for the value to reach equilibrium.